More than 32% of the mercury emitted in the United States to the atmosphere is from coal-burning utilities. Should further mercury control emissions from municipal solid waste and medical waste incinerators be mandated, the percentage of mercury released to the atmosphere from coal-burning utilities would greatly increase. A low concentration of mercury, on the order of 1 part per billion by volume (ppbv), is found in flue gas when coal is burned. The primary forms of mercury in the flue gas are elemental mercury and oxidized mercury (believed to be mercuric chloride).
Existing control technologies for the removal of mercury from flue gas include scrubbing solutions and activated carbon sorbents. An estimated 25% of power plants have wet scrubbers. Because mercuric chloride is soluble in water and elemental mercury is not, some mercuric chloride will be removed by the scrubbing process, whereas elemental mercury will not be removed by this process. Dry sorbents have the potential to remove both elemental and oxidized forms of mercury. Activated carbon is a dry sorbent that is suitable for removing contaminants, including mercury, from various gas streams. Activated carbon is typically obtained by heating carbonaceous material in the absence of air and then controlling an oxidation step with steam or carbon dioxide. This process results in a porous internal structure of the carbon that has good adsorptive properties. Activated carbons have been successfully applied for the control of mercury emissions from incinerators.
Carbon sorbents operate effectively over a limited temperature range, typically working best at temperatures below 300xc2x0 F. However, the major drawback of using activated carbon is that the projected annual cost for an activated carbon cleanup process is extremely high, not only because of the high cost of the sorbent, but also because of its poor utilization/selectivity for mercury. Most components of flue gas will adsorb on carbon and some are in competition with mercury for adsorption on carbon. Carbon-to-mercury weight ratios projected to remove mercury are in the range of 3,000:1 to 100,000:1. The cost of a ton of carbon can currently range from $1,000 to $3,000. Additionally, the commercially available sorbent must be manufactured off-site in a separate facility, shipped, and then stored at the power generation facility.
The system and method described here are for an inexpensive alternative to commercially available activated carbon. A thermally activated sorbent is obtained by retrieving partially combusted coal (raw coal is cheap at roughly $30/ton) from the combustion zone of the power plant""s combustion chamber. Once introduced to the flue gas, the sorbent reacts with mercury and removes it from the flue gas stream. The spent sorbent can be removed from the flue gas by the plant""s particulate collection device or by the installation of a particulate collection device just for the sorbent. The thermal activation is similar to the activation process of commercially available activated carbon. The carbon-to-mercury weight ratios required of the thermally activated sorbent produced in-situ are on the order of the ratio required to obtain mercury removal with activated carbon. The thermally activated sorbent produced in a manner described here is much less expensive than commercially available activated carbon and therefore is more easily and economically obtained and used on-site when its demand is increased. While activated carbon is a well known sorbent, the extraction of partially combusted coal for use as a sorbent elsewhere on site represents a significant improvement in the art of removing contaminants from flue gas.
Fly ash is the incombustible residue that remains unburned and is carried out of the combustion chamber. The fly ash has been used as a sorbent to remove extremely small particulate matter, fumes, or vapor phase contaminants from flue gas. U.S. Pat. Nos. 5,507,238 (xe2x80x9c""238xe2x80x9d) and 5,787,823 (xe2x80x9c""823xe2x80x9d) issued to Knowles describe the use of fly ash to remove contaminant species that can be sublimated, condensed or chemisorbed onto or into the fly ash particles. These patents describe the fly ash as having been modified by capture in a particulate separation device. The capture of the fly ash in a particulate separation device is said to modify the fly ash by coarsing its particle size distribution.
The ""238 and ""823 patents describe the use of fly ash that is taken from the exit port of the combustion chamber for use elsewhere in the system. The in-situ method for producing a carbon sorbent described here couples on-site sorbent production with the injection of the sorbent into the duct work of a utility. Carbon structures formed and present in the combustion zone of pulverized coal plants are much more reactive for the removal of mercury from downstream flue gas than residual carbon found on fly ash which had a longer residence time within the combustion chamber and where oxidation of carbon is more nearly complete.
Fly ash may sometimes contain unburned carbon that may be separated from the fly ash and activated for the removal of contaminants from flue gas. However, such processes cannot alter the characteristics of the carbon extracted from fly ash without significant physical and/or chemical treatments. Thus, the thermally activated solids of the system and method described here will outperform fly ash of equivalent carbon content. The better performance is due to the inherent nature of the complex carbon structures present in the thermally activated solids, such as residual sulfur content, etc. that create a greater reactivity toward the removal of mercury from flue gas. Therefore, the system and process described here represent significant improvements in the art of flue gas treatment for the removal of mercury.
An object of this invention is to provide a system and method of producing a thermally activated carbon sorbent by extracting a stream of partially combusted carbon from the combustion chamber of a coal-fired power plant.
A further object of this invention is to use the thermally activated sorbent at the power plant where it was produced to remove mercury from flue gas by injecting the sorbent into the duct work of the facility downstream from the exit port of the combustion chamber. The spent sorbent can be collected in the plant""s particulate collection system or in a particulate collection system dedicated just for the sorbent.
The system and method described here represent an inexpensive alternative to commercially available activated carbon for removing mercury from the flue gas of a coal fired power plant. Mercury removal is by adsorption onto a thermally activated sorbent produced in-situ at the power plant. To obtain the thermally activated sorbent, at least one lance, called a xe2x80x9cthiefxe2x80x9d, is inserted into a location within the combustion zone of the combustion chamber and extracts a mixture of semi-combusted coal and gas. The semi-combusted coal has adsorptive properties suitable for the removal of elemental and oxidized mercury. The mixture of semi-combusted coal and gas is separated into a stream of gas and semi-combusted coal that has been converted to a stream of thermally activated sorbent. The separated stream of gas can be recycled to the combustion chamber. The thermally activated sorbent is injected into the duct work of the power plant at a location downstream from the exit port of the combustion chamber. Mercury within the flue gas contacts and adsorbs onto the thermally activated sorbent. The used sorbent-mercury combination is removed from the flue gas by a particulate collection system.